Fix tests under Python 3.12

There's a test that checks whether `import setup` works; our setup.py
requires setuptools.

Before Python 3.12, setuptools was preinstalled in all the virtualenvs.
Virtualenv 20.23.0 changed this:

  - Do not install wheel and setuptools seed packages for Python 3.12+.

See pypa/virtualenv#2487 for details and
rationale.

Author: mgedmin

tox.ini

@@ -2,6 +2,8 @@
 envlist = py37, py38, py39, py310, py311
 
 [testenv]
+deps =
+    setuptools
 commands =
     python tests.py {posargs}
 
@@ -12,6 +14,7 @@ commands =
 
 [testenv:coverage]
 deps =
+    {[testenv]deps}
     coverage
 commands =
     coverage run tests.py